Since the birth of the semiconductor industry, photolithography has been used for forming the various components that make up integrated circuits. The continued increase in the density of components that can be placed on a chip has been largely due to advances in photolithography associated with using radiation of ever decreasing wavelengths. As long as the minimum size (critical dimension) of the components was greater than the wavelength of the radiation being used to expose the photoresist, advances in the art did not require any changes in the masks used other than to reduce the sizes of the components.
Inevitably, a time came when the critical dimensions got to be less than about half the wavelength of the radiation being used, so radiation of lower wavelength had to be substituted. Eventually, critical dimensions reached, and then went below, the lower limit of optical lithography where conventional optics and resists can still be used (about 180 nm). Although it has been demonstrated that X-ray lithography is capable of producing patterns whose critical dimension is one or two orders of magnitude less than that, cost considerations have continued to drive conventional lithography to seek ways to image sub-optical critical dimensions while still using optical techniques.
When the wavelength of the imaging radiation gets to be larger than the critical dimension, the effects of diffraction, though always present, become prominent enough to introduce noticeable distortions into the images projected relative to their original shapes on the imaging mask. These distortions are particularly sensitive to the distances between the various features in the pattern and are therefore referred to as `proximity effects`.
Another problem associated with photolithography at wavelengths close to the critical dimensions is depth of focus (DOF). In particular, when the DOF is less than the thickness of the resist being exposed, image sharpness will be lost. If light rays had no width, they would focus in a plane that was infinitely thin. In practice, because of diffraction effects, the best that can be achieved is a blur circle. As long as rays from the same point on the object are within the blur circle, they are considered to be in focus.
When resolution is not a problem, DOF can be increased by restricting the incoming light to the center of the lens. This then reduces the angle of the light cone so that focused rays travel further before leaving the blur circle. When resolution is also a consideration, this solution is no longer acceptable. One way to increase DOF without having to forego contributions from higher order diffraction maxima is to use off-axis illumination. This narrows the cone of illumination, thereby increasing DOF, while at the same time bringing more of the higher order diffraction maxima close to the center of illumination.
DOF of any particular object is, however, also affected by its proximity to other objects within the same pattern. The average degree of proximity between objects for a full pattern is conveniently expressed as a duty ratio which is defined as (total dark width)/(total clear width). Thus, DOF varies as a function of the duty ratio. The exact relationship between DOF and duty ratio is very complex but, most commonly, the higher the duty ratio (i.e. the more dark area), the greater the DOF.
Most prior art work to increase DOF has been directed towards patterns that are made up largely of lines, with the object of bringing isolated and densely packed lines simultaneously into focus (by increasing the DOF). The present invention is directed towards bringing both isolated and densely packed shapes that define contact holes into simultaneous focus. Because of the very complex nature of the optics that are involved, a solution that works for lines will not necessarily work for contact holes, and vice versa.
As it is an integral part of the present invention, it is important that we clarify our use of the term `pupil plane`. In a lens system, a pupil plane is defined as one through which light from all parts of the object passes while being at the same time completely mixed, with no preferential special separation. In a projection system, the central plane of the lens itself meets this definition so is an example of a pupil plane. A stop inserted in front of the lens also meets this definition, forming an entrance pupil, while the image of the stop defines the plane of the exit pupil.
Referring now to FIG. 1, the basic components that make up a projection system for photolithography are schematically illustrated. Light beam 11 is condensed by illuminator lens 12 so that reticle 13, that includes feature 19, is uniformly illuminated. Most of the original beam passes straight on as the zero order diffraction maximum 14, while first order diffraction maxima 15 and higher order maxima 16 are diffracted off to the side. These are then focused by projection lens 17 onto focal plane 18. Since no information (other than overall brightness) is contained in the zero order maximum, it is imperative that at least some of the higher order beams contribute to the image. This necessarily widens the angle of the focusing cone resulting, as seen above, in a reduced DOF.
In FIG. 2, the basic setup of FIG. 1 has been modified so that light beam 11 is blocked from the center of illuminator lens 12, being limited to coming in obliquely (off-axis). The result of this is that the zero order diffraction maximum 14 is forced over to the edge of projection lens 17 while 1.sup.st order maximum 15 passes (approximately) through the center of the projection lens, thereby allowing a narrower angle for the focusing cone, with a corresponding increase in DOF.
In FIG. 3, a different modification of the basic setup of FIG. 1 has been introduced. This is the placement of phase-type filter 31 at the pupil plane of the projection lens 17. Its effect is to change the phase of the first and higher order diffraction maxima by 180.degree. relative to that of the zero order maximum 14. This results in an increase of DOF for dense patterns.
A routine search of the prior art was conducted but no prior art that provides the precise solution (to the problem of simultaneously imaging both dense and isolated contact hole shapes) was encountered. Several references of interest were, however, found. For example, in U.S. Pat. No. 5,691,803 Song et al. show a system in which both quadrupole and annular illumination is used while in U.S. Pat. No. 5,663,785 Kirk et al. show a modified pupil filter that provides a spinning diffraction filter placed in a stepper to provide annular illumination on a time averaged basis.
In U.S. Pat. No. 5,863,712 Von Bunau et al. show a pupil filter with a variable amplitude transmittance while Shiraishi, in U.S. Pat. No. 5,610,684, shows an exposure system with an optical correction plate that differs from that of the invention. This patent provides a description of the commercially available Nikon Shrinc filter. In a paper entitled "Quarter micron lithography system with oblique illumination and pupil filter" (SPIE vol. 2197 pp. 854-857 1994) Orii et al. conclude that if oblique illumination is to be used in conjunction with a pupil filter, then dipole illumination should be used. In a European patent (0 562 133 A1) Sandstrom that off-axis illumination should be optimized for line patterns that run at a 45.degree. angle relative to the bulk of the wiring.